Resistivity logging, which measures the electrical resistivity of formations surrounding an earth borehole, is a commonly used technique of formation evaluation. For example, porous formations having high resistivity generally indicate the presence of hydrocarbons, while porous formations having low resistivity are generally water saturated. In so-called "wireline" well logging, wherein measurements are taken in a well bore (with the drill string removed) by lowering a logging device in the well bore on a wireline cable and taking measurements with the device as the cable is withdrawn, there are several techniques of resistivity logging which use elements such as electrodes or coils. Various arrangements of electrodes, on the logging device and at the earth's surface, have been utilized to measure electrical currents and/or potentials from which formation resistivity can be derived. For example, button electrodes have been employed on a pad which is urged against the borehole wall. These electrodes have been used to obtain azimuthal resistivity measurements, and focusing techniques have been employed to obtain resistivity measurements that have substantial lateral extent into the formations and provide relatively high vertical resolution resistivity information.
Various techniques for measuring resistivity while drilling have also been utilized or proposed. Techniques employed in wireline logging may or may not be adaptable for use in a measurement-while-drilling equipment. The borehole presents a difficult environment, even for wireline logging, but the environment near the well bottom during drilling is particularly hostile to measuring equipment. For logging-while-drilling applications, the measuring devices are housed in heavy steel drill collars, the mechanical integrity of which cannot be compromised. Measurement approaches which require a substantial surface area of electrically insulating material on the surface of a drill collar housing are considered impractical, since the insulating material will likely be damaged or destroyed. This is particularly true for measuring structures that would attempt to attain intimate contact with the newly drilled borehole wall as the drill string continues its rotation and penetration, with the attendant abrasion and other stresses.
One resistivity measuring approach is to utilize a plurality of toroidal coil antennas, spaced apart, that are mounted in insulating media around a drill collar or recessed regions thereof. A transmitting antenna of this nature radiates electromagnetic energy having a dominant transverse magnetic component, and can use the electrically conductive body of the drill collar to good advantage, as described next.
In U.S. Pat. No. 3,408,561 there is disclosed a logging-while-drilling system wherein a receiving toroidal coil is mounted in a recess on a drill collar near the drill bit and a transmitting toroidal coil is mounted on the drill collar above the receiver coil. The drill collar serves as part of a one-turn "secondary winding" for the toroidal antennas, the remainder of such "secondary winding" including a current return path through the mud and formations. The voltage induced in the receiver toroidal coil provides an indication of the resistivity of formations around the drill bit. U.S. Pat. No. 3,305,771 utilizes a similar principle, but employs a pair of spaced-apart transmitting toroidal coils and a pair of spaced-apart receiving toroidal coils between the transmitting toroidal coils.
As generally described in the prior art, a transmitter toroidal coil mounted on a drill collar induces current in the drill collar which can be envisioned as leaving the drill collar, entering the formations below the transmitter coil, and returning to the drill string above the transmitter coil. Since the drill collar below the transmitter coil is substantially an equipotential surface, a portion of the current measured by a lower receiver toroidal coil mounted near the drill bit tends to be laterally focused. This can provide a "lateral" resistivity measurement of formations adjacent the drill collar. Also, a portion of current leaving the drill stem below the receiver coil (mostly where the bit contacts the formations) provides a "bit resistivity" measurement; that is, a measurement of the resistivity of the formations instantaneously being cut by the bit. [See, for example, the above-identified U.S. Pat. Nos. 3,408,561 and 3,305,771, and publications entitled "A New Resistivity Tool For Measurement While Drilling", SPWLA Twenty-Sixth Annual Logging Symposium (1985) and "Determining The Invasion Near The Bit With The MWD Toroid Sonde", SPWLA Twenty-Seventh Annual Logging Symposium (1986).] Thus, the prior art indicates that a measurement-while-drilling logging device using toroidal coil transmitting and receiving antennas can be employed to obtain lateral resistivity measurements and/or bit resistivity measurements.
Reference can also be made to the following which relate to measurement-while-drilling using electrodes and other transducers: U.S. Pat. No. 4,786,874, U.S. Pat. No. 5,017,778, and copending U.S. patent application Ser. No. 525,268 filed May 16, 1990, now U.S. Pat. No. 5,130,950 assigned to the same assignee as the present application.
Resistivity measurements obtained using transmitting and receiving toroidal coils on a conductive metal body are useful, particularly in logging-while-drilling applications, but it would be desirable to obtain measurements which can provide further information concerning the downhole formations; for example, lateral resistivity information having improved vertical resolution, azimuthal resistivity information, and multiple depths of investigation for such resistivity information. It is among the objects of the present invention to devise equipment which can provide such further resistivity measurement information.
In logging-while-drilling applications, various schemes have been proposed for transmitting the measurement information to the surface of the earth. A number of these schemes involve using a toroidal coil antenna to radiate electromagnetic energy having a transverse magnetic component from downhole to the earth's surface, or to repeaters along the drill string which receive, boost, and re-transmit the signals using further toroidal coil transmitters. As in the systems first described above which utilize toroidal coils for obtainment of resistivity measurements, the drill string is used as a current carrier. Reference can be made, for example, to U.S. Pat. Nos. 3,186,222, 3,967,201, 4,578,675, 4,725,837, 4,739,325, and 4,839,644. In the U.S. Pat. No. 4,578,675 there is disclosed a logging-while-drilling apparatus which utilizes toroidal coil antennas to obtain bottom-hole resistivity measurements and employs one of these antennas, on a time-sharing basis, for two-way communication with equipment at the surface of the earth. The communication may be via passive or active repeater units further uphole. In general, downhole/surface electromagnetic telemetry approaches which use the drill string as a current carrying component (and, typically, the mud and the formations as a return current path) have intrinsic limitations. The mud conductivity and the conductivity and heterogeneity of the surrounding formations will affect the signal, and the need for boosters or repeaters is inconvenient and expensive.
For various reasons, the approach that has been the most successful for logging-while-drilling communication between the well bottom and the earth's surface has been so-called mud pulse telemetry. Briefly, pressure pulses (or acoustic pulses) modulated with the information to be conveyed, are applied to the mud column [typically downhole, for communication to the surface, although two-way communication is also used], and received and demodulated uphole.
A downhole mud telemetry subassembly typically includes the equipment for controlling data communication with the surface and for applying modulated acoustic pulses to the mud. When a measurement subassembly (e.g. one measuring formation parameters and/or other parameters concerning drilling such as downhole weight on bit or direction and inclination of the borehole) is housed in a drill collar that is mounted adjacent the downhole mud telemetry subassembly, a wiring connector can be provided for electronic connection between these subassemblies. The nature of the drill collar sections housing these units, the typical threaded mechanical connections therebetween, and the stresses to which the connections are subjected, render the connection of wires somewhat inconvenient, but such connections are commonly implemented. A larger problem arises, however, when a desired bottom hole arrangement of telemetry equipment, measurement collars, stabilizer collars, etc. involves separation between the mud telemetry subassembly and one or more measurement subassemblies that are intended to communicate therewith. Under such circumstance, wiring buses and connectors may be provided for local electronic communication between the measurement subassembly and the downhole mud telemetry subassembly, but the requirement for crossing other drill collar sections and joints is disadvantageous. The problem is exacerbated when the relative placements of a particular measurement subassembly (or subassemblies) with respect to the downhole mud telemetry subassembly is not known a-priori and is decided spontaneously at the well site, as is often the case in modern drilling operations.
It is therefore among the further objects of the present invention to provide improvement in the efficiency and flexibility of communications in logging-while-drilling systems.